The present invention is directed to physical structures and methods for separating hematocrit out of small volume whole blood samples leaving merely the plasma or plasma containing a substantially reduced partial volume of hematocrit. The present invention is particularly directed to such structures having no moving parts which subsequent to separation of the plasma would facilitate contacting the reduced hematocrit content plasma with a dry reagent to permit an accurate detection of an analyte.
Many diagnostic tests are carried out in the clinical field utilizing a blood sample. It is desirable, when possible, to use a very small volumes of blood, often no more than a drop or two. Capillary structures are often employed when handling such small volumes of blood or other fluids. The presence of the hematocrit in the blood sample often interferes with accurate testing and so the removal of, or reduction in concentration of, the hematocrit in the sample, leaving a reduced hematocrit content plasma for testing, is often desirable or even necessary. The removal of the hematocrit is often done using a filter. An example of such a filter device employing capillary structures is described in Hillman, et al., U.S. Pat. Nos. 4,753,776 and 5,135,719. Other devices employing capillary structures to handle whole blood samples are disclosed in McDonald, et al., U.S. Pat. No. 5,039,617; Hillman, et al., U.S. Pat. No. 4,963,498; and Columbus, U.S. Pat. No. 4,271,119.
While such filter devices generally perform satisfactorily, many filter materials tend to absorb a significant portion of the plasma from the blood sample thus leaving only a small volume of the reduced plasma for analytical testing. As the total volume of the sample is diminished, the proportion of the plasma fraction that is absorbed by the filter tends to increase leaving even smaller volumes for testing. It is therefore desirable to construct alternative means for removing hematocrit from whole blood that would be usable on very small sample volumes.